1. Field of the Invention
The present invention relates generally to semiconductor integrated circuits, and more specifically to the implantation of channel-stop regions used in the oxidation process of field oxide.
2. Description of the Prior Art
In fabricating semiconductor circuits, the integrity of various structures on the circuit is of great importance. Undesirable impurities in the wrong portions of a circuit can adversely affect its function.
One type of structure commonly found on a semiconductor integrated circuit is a transistor. Field effect transistors can be either n-channel or p-channel, and both types of transistors can be formed on one integrated circuit. Field oxide regions are formed to separate the transistors. In this process regions of silicon dioxide are selectively grown on the substrate to provide isolation between dopant wells. As known in the art, transistors will be defined in regions without field oxide. These regions are known as active regions.
The process of growing the field oxide involves depositing and patterning insulating layers which protect the active region from oxidation. In one technique, an oxide layer is grown over the substrate, followed by depositing a polycrystalline silicon layer over the device. A layer of silicon nitride is then deposited over the polycrystalline silicon layer. Using a photoresist mask, an anisotropic etch is performed to create an opening through the insulating layers to expose a portion of the substrate.
The exposed portion of the substrate is implanted with a channel-stop region. The purpose of the channel-stop is to prevent latchup caused by turning on parasitic bipolar transistors. Following implantation of the channel-stop field oxide is grown on the exposed portion of the substrate.
As known in the art, oxidation of the substrate is performed using high temperatures. The high temperature causes the channel-stop implant to diffuse both laterally and vertically into the substrate. Migration of the channel-stop implant into an active region can cause the transistor or the integrated circuit to be defective. Those skilled in the art will recognize that the defects are the result of edge effects, which can include a shift in the threshold voltage.
It would be desirable to provide a method for reducing the diffusion of the channel-stop region during formation of field oxide in integrated circuit.